Hi Steve,
We do audio sweetening roundtrips between FCP PAL projects and PT on a regular basis and this is the first time I hear about such a problem. We also do film conforming projects in true 24p on a regular basis, no problems either. Maybe we just have been lucky, or something has gone wrong in your workflow.
We either use straight OMF roundtrips (simple FCP timelines) or Automatic Duck (complex FCP timelines) to go to PT and back. I think that is the correct way to work between FCP and PT and this works very well for us. I am not aware of any automatic conversions done by FCP, unless you have an FCP project where the settings do not match. But that's a problem created by the operator, not the system. Anyway, it's good of you to report this issue you have had so we can all learn from it.
Best wishes,
Ronny